This invention relates to a method and apparatus for detecting the presence of a gas, and more particularly to a gas sensor that detects a gas by detecting changes in optical properties of a semiconductor in response to adsorption of the gas on the semiconductor surface.
Many pollutants in the atmosphere are potentially harmful, a number of which have been categorized as most urgent to detect and eliminate. They include particulates (SiO.sub.2, asbestos), various oxides such as NO.sub.x, SO.sub.2, and CO, CH.sub.4, Pb, H.sub.2 S, mercaptans, and organic sulfides, as well as visibility reducing particles such as dust and sand. The levels of various individual pollutants have varied over the past 20 years. For example, carbon monoxide and hydrocarbons have declined, while the nitrogen oxides, which are a major health hazard, are rising sharply. Thus, a need exists for methods and apparatus for accurately measuring and monitoring gases and pollutants in the atmosphere.
Nonheated gas sensors have been devised that avoid some of the dangers associated with heated gas sensors. For example, U.S. Pat. No. 4,661,320 to Ito et al. describes a gas sensor comprising a device by which optical absorption is changed in the presence of hydrogen gas (or a hydrogen-containing compound) and a detecting apparatus is used for detecting the change in optical absorption. The device is made of a laminate of catalytic metal, which causes dissociation of gas molecules when a hydrogen gas is present. As a result, hydrogen protons are injected into a solid compound underlying the metal laminate, causing a corresponding change in the compound's rate of optical absorption. The change in optical absorption is detected by apparatus sensitive to a change in intensity of light transmitted through the device, thereby indicating the presence of a hydrogen gas or hydrogen gas compound.
The injection of hydrogen protons into the solid compound is a chemical reaction, which requires that the protons be energized to diffuse and penetrate and cross the potential barrier to reach a position inside the material. Therefore, a disadvantage of Ito's technique is that energy need be input to sustain the chemical reaction and also to reverse the reaction and restore the transparency of the solid compound.